Fluorite

ABSTRACT

A fluorite having all the more excellent laser durability compared to a conventional fluorite is provided. A fluorite is proposed, in which the standard deviation of the surface areas of the Voronoi regions in a diagram from a Voronoi segmentation of the distribution of etch-pits in the (111) plane is 6,000 μm 2  or less, or, in which the standard deviation of the distances of the Delaunay edges in a diagram from a Delaunay segmentation of the distribution of etch-pits of the (111) plane is 80 μm or less.

TECHNICAL FIELD

The present invention relates to a fluorite (CaF₂, calcium fluoride)that can be used, for instance, as an optical lens and a lens materialused in semiconductor lithography or the like.

TECHNICAL BACKGROUND

Having special partial dispersion characteristics (: anomalous partialdispersion; Abbe number: 95) in addition to having extremely smallchromatic dispersion, low refractive index and dispersion ratio comparedto generic optical glasses, crystals of fluorite (CaF₂ crystals) areused broadly in apochromatic lenses (apochromats), window plates ofinfrared analyzers, excimer lasers and the like, TV camera lenses andmicroscope lenses, lenses of semiconductor lithography (includingsteppers, scanners and the like) devices, which are devices fortransferring microscopic patterns onto wafers, and the like.

Among them, regarding steppers (reduction projection-type exposuredevices), which assume the miniaturization process in semiconductorlithography devices, shortening of the light source wavelength has beenproceeding in order to raise the resolving power, and steppers have beendeveloped, which use for the light source an excimer laser serving as ahigh output laser oscillating in the ultraviolet region, andconcomitantly to this, fluorite (CaF₂, calcium fluoride) has beendrawing attention as a lens material suitable thereto. That is to say, afluorite is characterized by the transmittance being high for lightbeams in a wavelength region called vacuum ultraviolet region, such asfrom a KrF laser (wavelength: 248 nm) or an ArF laser (wavelength: 193nm), among the excimer laser beams. As one factor influencing theoptical characteristics of a fluorite (calcium fluoride), refractiveindex homogeneity, or sub-boundary, which is a portion where dislocationhas accumulated, can be cited.

As conventional technique focusing on such refractive index homogeneity,using a calcium fluoride (for instance Patent Document 1) having arefractive index homogeneity of 5 ppm (that is to say, 5,000 ppb) orlower and a birefringence of 10 nm/cm or lower, or a calcium fluoride(for instance Patent Document 2) having a refractive index homogeneityof 3 ppm (that is to say, 3,000 ppb) or lower and a birefringence of 2nm/cm or lower in the exposure optical system has been proposed.

In addition, as a novel fluorite provided with a homogeneous CaF₂crystal with little strain birefringence (distortion), a fluorite isdescribed in Patent Document 3, in which, when the light incidence planeis the (100) plane in a crystal substrate having parallel planes in the<100> direction, the mean value of the strain birefringence value perthickness at 633 nm wavelength is 0.4 nm/cm to 1.8 nm/cm, and thedifference (PV) between the maximum value and the minimum value of thestrain birefringence values per thickness at 633 nm wavelength is 4.0nm/cm or less.

Meanwhile, lens materials for an excimer laser are sometimes damaged byso-called photodamages, in which a color center is formed within thematerial while being irradiated by a light beam such as from a laser,provoking a local change in refractive index due to a decrease intransmittance or absorption heating. Furthermore, when irradiated by astrong laser beam, not only the photodamages described above, but alsodestruction due to heat stress induced by absorption heating, or,damages due to insulation destruction, or the like, by the strongphotoelectric field from the laser beam, are sometimes sustained, suchthat laser durability is one important evaluation item in this speciesof optical materials.

As conventional technique focused on such laser durability, for instancein Patent Document 4, a UV-compatible fluorite, in which the internaltransmittance is 99.5%/cm or greater in a wavelength region of 150 nm orlonger but 300 nm or shorter when irradiated with a pulsed laser beam inthe ultraviolet light wavelength region for a number of pulses of 10⁴ ormore but 10⁷ or less at an energy density of 1 mJ/cm²/pulse or more but50 mJ/cm²/pulse or less, is described as a fluorite with excellentdurability against ultraviolet light from KrF and ArF excimer lasers orthe like.

In addition, in Patent Document 5, a calcium fluoride crystal, in which,after irradiating γ-ray at a dose of 1×10⁵ R/hour for one hour, theamount of reduction of the internal transmittance per 10 mm thickness at260 to 280 nm wavelength is 8% or lower before irradiation, is describedas a calcium fluoride crystal with excellent durability against laserssuch as excimer lasers.

PRIOR ART REFERENCES Patent Document

-   [Patent Document 1] Japanese Patent Application Laid-open No.    H8-5801-   [Patent Document 2] Japanese Patent Application Laid-open No.    H10-270351-   [Patent Document 3] Japanese Patent Application Laid-open No.    2008-156164-   [Patent Document 4] Japanese Patent Application Laid-open No.    2001-41876-   [Patent Document 5] Japanese Patent Application Laid-open No.    2000-211920

DISCLOSURE Problems to be Solved by the Invention Summary of theInvention

The present invention provides a novel fluorite with all the moreexcellent laser durability compared to a fluorite of the prior art.

The present invention provides a fluorite, in which the standarddeviation of the surface areas of the Voronoi regions in a diagram froma Voronoi decomposition of the distribution of etch-pits in the (111)plane, that is to say, the standard deviation of the surface areas ofthe Voronoi regions (noted “Voronoi surface areas”) when a Voronoidiagram has been defined (via Voronoi segmentation) with each etch-pitserving as a generatrix in the distribution of etch-pits obtained byetching the (111) plane of a fluorite, is 6,000 μm² or less, or, afluorite, in which the standard deviation of the distances of theDelaunay edges in a diagram from a Delaunay decomposition of thedistribution of etch-pits of the (111) plane, that is to say, thestandard deviation of the lengths of the Delaunay edges (noted “Delaunaydistances”), in a Delaunay diagram (via Delaunay segmentation) definedby joining together Delaunay points when each etch-pit serves as aDelaunay point in the distribution of etch-pits obtained by etching the(111) plane of a fluorite, is 80 μM or less, or, a fluorite satisfyingboth of these conditions. For purposes of illustration: etch-pits 53 areidentified in FIGS. 7, 10, 13, and 16; Delaunay edges 56 are identifiedin FIGS. 8, 11, 14, and 17; and Voronoi regions 59 are identified inFIGS. 9, 12, 15, and 18.

In prior art, the method of evaluating etch-pit density (EPD) has beenknown as a technique for evaluating dislocations and crystal defects ina crystal substrate. That is to say, it is a method, in which a cleansurface obtained by cleavage, or the like, of a crystal is immersed inan adequate etching solution thereby forming corroded pores (etch-pits),the crystal surface is magnified by a microscope, or the like, and theetch-pits present within this enlarged image are evaluated by beingconverted into counts per unit surface area.

However, when evaluating etch-pit density (EPD) in this way, there isthe problem that, whether the etch-pits are present densely alined orare present uniformly dispersed, since the value is the same as far asthe density is concerned, suitability of defect distribution(distribution of etch-pits) in the CaF₂ crystal cannot be evaluatedaccurately, and correlation with laser durability is not identified. Incontrast, when a fluorite is evaluated as in the present invention withthe standard deviation (variation) of the Voronoi surface areas or theDelaunay distances in the distribution of the etch-pits, representingthe suitability of the distribution of the etch-pits more accuratelybecomes possible, and correlation with laser durability has also becomeidentifiable.

Consequently, since the fluorite according to the present invention is ahomogeneous fluorite, in which there are little dislocations andsub-boundary structures, and in particular having excellent laserdurability, it can be used suitably as a lens material such as, forinstance, TV camera lens, microscope lens, window material for infraredanalysis, lens used in semiconductor lithography devices, in particular,a lens material for a stepper of an exposure apparatus, or the like,using a laser as a light source in the ultraviolet or vacuum ultravioletwavelength region such as an ArF (argon fluoride) excimer laser exposureapparatus and an F₂ (fluorine) excimer laser exposure apparatus, whichrequire optical characteristics of high degrees.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Cross-sectional view showing an example of heat-treatment ovenused in the present invention.

FIG. 2 Magnified cross-sectional view of relevant portion showing eachcompartment-walled vessel magnified centered thereon in FIG. 1.

FIG. 3 Graph showing the relationship between the color centerabsorption intensity induced by γ-ray irradiation, which is analternative property to laser durability, and the standard deviation ofVoronoi surface areas for the samples obtained in Examples 1 to 8 andComparative Examples 1 to 5.

FIG. 4 Graph showing the relationship between the color centerabsorption intensity induced by γ-ray irradiation, which is analternative property to laser durability, and the standard deviation ofDelaunay distances for the samples obtained in Examples 1 to 8 andComparative Examples 1 to 5.

FIG. 5 View comparing the etch-pit images (Etch-pit observation images),the Delaunay diagrams and the Voronoi diagrams of the samples obtainedin Example 1 and Comparative Example 1 aligned at the top and the bottomrespectively.

FIG. 6 View comparing the etch-pit images (Etch-pit observation images),the Delaunay diagrams and the Voronoi diagrams of the samples obtainedin Example 2 and Comparative Example 2 aligned at the top and the bottomrespectively.

FIG. 7 Etch-pit image (Etch-pit observation image) of the sampleobtained in Example 1.

FIG. 8 Delaunay diagram of the sample obtained in Example 1.

FIG. 9 Voronoi diagram of the sample obtained in Example 1.

FIG. 10 Etch-pit image (Etch-pit observation image) of the sampleobtained in Comparative Example 1.

FIG. 11 Delaunay diagram of the sample obtained in Comparative Example1.

FIG. 12 Voronoi diagram of the sample obtained in Comparative Example 1.

FIG. 13 Etch-pit image (Etch-pit observation image) of the sampleobtained in Example 2.

FIG. 14 Delaunay diagram of the sample obtained in Example 2.

FIG. 15 Voronoi diagram of the sample obtained in Example 2.

FIG. 16 Etch-pit image (Etch-pit observation image) of the sampleobtained in Comparative Example 2.

FIG. 17 Delaunay diagram of the sample obtained in Example 2.

FIG. 18 Voronoi diagram of the sample obtained in Example 2.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail; however, the scope of the present invention is not limited tothe embodiments described below.

<The Present Fluorite>

The fluorite according to the present embodiment (hereinafter referredto as “the present fluorite”) is a fluorite, in which the standarddeviation of the surface areas of the Voronoi regions in a diagram froma Voronoi segmentation of the distribution of etch-pits in the (111)plane is 6,000 μm² or less, or, in which the standard deviation of thedistances of the Delaunay edges in a diagram from a Delaunaysegmentation of the distribution of etch-pits of the (111) plane is 80μm or less, or, satisfying both of these conditions.

(Standard Deviation of the Voronoi Surface Areas)

The standard deviation of the surface areas of the Voronoi regions in adiagram from a Voronoi segmentation of the distribution of etch-pit inthe (111) plane means the standard deviation of the surface areas of theVoronoi regions (“Voronoi surface areas”) when a Voronoi diagram hasbeen defined (via Voronoi segmentation) with each etch-pit serving as ageneratrix in the distribution of etch-pits obtained by etching the(111) plane of a fluorite. For the present fluorite, it is desirablethat the standard deviation of the Voronoi surface areas is 6,000 μm² orless. If the standard deviation is 6,000 μm² or less, the fluoritebecomes one having excellent laser durability.

From such point of view, for the standard deviation to be 4,000 μm² orless is more desirable, and in particular, to be 3,000 μm² or less isall the more desirable. Regarding the lower limit value, as there is noparticular limitation, it is most preferably 0 μm²; however,realistically, 1,000 μm² or greater is desirable.

(Standard Deviation of the Delaunay Distances)

The standard deviation of the distances of the Delaunay edges in adiagram from a Delaunay segmentation of the distribution of etch-pits ofthe (111) plane means the standard deviation of the lengths of theDelaunay edges (“Delaunay distances”) in a Delaunay diagram (viaDelaunay segmentation) defined by joining together Delaunay points wheneach etch-pit serves as a Delaunay point in the distribution ofetch-pits obtained by etching the (111) plane of a fluorite. For thepresent fluorite, it is desirable that the standard deviation of theDelaunay distance is 80 μm or less. If the standard deviation is 80 μmor less, the fluorite becomes one having excellent laser durability.

From such point of view, for the standard deviation to be 60 μm or lessis more desirable, and in particular, to be 50 μm or less is all themore desirable. Regarding the lower limit value, as there is noparticular limitation, it is most preferably 0 μm; however,realistically, 30 μm or greater is desirable.

<Production Method for the Present fluorite>

For instance, a CaF₂ crystal obtained by being grown via aconventionally well-known method can be heat-treated by providing afluoride gas trap layer containing a fluoride gas-adsorbing materialthrough compartment walls in the periphery of the fluorite crystal,thereby obtaining the present fluorite. However there is no limitationto such a production method.

A more detailed description follows.

It suffices that the crystal growth step adopts a conventionallywell-known method.

For instance, CaF₂ raw materials in powder form, or a mixture of thisand lead fluoride (PbF₂) serving as a scavenger, is filled into acrucible, this crucible is installed inside a crystal growth apparatus,evacuation is performed with a vacuum exhaust system until the degree ofvacuum inside the crystal growth apparatus reaches on the order of1×10⁻³ to 10⁻⁴ Pa, and the crucible is heated with a heater to melt theraw materials filled into the crucible. Here, while gas generated byreacting impurities to fluorite with the scavenger and gas adsorbed onthe inner walls of the oven and the crucible desorb concomitantly to therise in temperature of the crucible, these are evacuated rapidly outsidethe system by the vacuum exhaust system of the crystal growth apparatus.After the raw materials are melted, the temperature inside the oven ismaintained at a constant temperature while maintaining the desireddegree of vacuum. Thereafter, when the crucible is lowered verticallydownward gradually at a speed of on the order of 0.1 mm/time to 3mm/time, the melt inside the crucible starts solidifying from near thecrucible bottom portion, the crystal becomes extended and growngradually concomitantly to the lowering of the crucible. At the stagewhere the entirety of the melt inside the crucible has solidified, thelowering of the crucible is terminated, and while slowly cooling with aheater, the crucible is cooled to about room temperature, allowing aCaF₂ crystal in ingot form to be grown.

However, there is no limitation to such a crystal growth method.

Cutting the CaF₂ in ingot form obtained in this way so that the surfacein a given direction appears and subjecting it to the followingheat-treatment step is adequate. For instance, cutting into a disk-shapeof on the order of 200 mm in diameter and on the order of 40 mm inthickness and subjecting it to a heat-treatment step is adequate.

Next, for instance as shown in FIG. 1, it is important that the CaF₂crystal obtained in the crystal growth step is heat-treated by providinga fluoride gas trap layer 5 containing a fluoride gas-adsorbing materialthrough compartment walls in the periphery of the CaF₂ crystal 50 to beheat-treated.

Here, the heat-treatment oven shown in FIG. 1 will be described indetail.

In FIG. 1, numeral 1 is a vacuum vessel, numeral 2 is a heater, numeral3 is an annealing case, numeral 4 is a support vessel, numeral 5 is afluoride gas trap layer, numeral 6 is a compartment-walled vessel,numeral 7 is a scavenger or a fluoridated agent, and numeral 50 is aCaF₂ crystal.

This heat-treatment oven is surrounded by a vacuum vessel 1 that mayretain the interior in an airtight state, and is constituted in a waythat allows the atmosphere inside the vacuum vessel 1 to be adjusted toa predetermined state as well as the temperature inside the vacuumvessel 1 to be controlled with high accuracy according to a determinedtemperature profile.

To adjust the atmosphere of the vacuum vessel 1 to a predeterminedstate, for instance, it is adequate to adjust the atmosphere byevacuating the gas inside the vacuum vessel 1 with an exhaust system andintroducing a predetermined gas in suitable amounts with an inletsystem.

In addition, to control the temperature of the vacuum vessel 1 with highaccuracy according to a determined temperature profile, it is adequate,for instance, to install temperature sensors at suitable heights nearthe external walls of a support member 4 installed inside the vacuumvessel 1, for instance, at each height among an upper layer portion, amiddle layer portion and a lower layer portion, and controlling withthese temperature sensors and a temperature controller the temperaturesof a plurality of heaters 2 a, 2 a . . . disposed along the side wallsof the vacuum vessel 1.

The vacuum vessel 1 is formed from stainless or the like, inside ofwhich is installed an annealing case 3.

The annealing case 3 is a vessel that fills a role for the purpose ofsupporting the support vessel 4 for retaining or supporting theheat-treatment subject, that is to say, the CaF₂ crystal, and can beformed from a carbon material. In addition, it is also a vessel thatfills a role for the purpose of distributing the surrounding temperatureof the support vessel 4 into an even heat.

A plurality of support vessels 4 stacked from top to bottom are housedinside this annealing case 3.

The support vessel 4 is a vessel that fills a role for the purpose ofsupporting the CaF₂ crystal, which is the heat-treatment subject,comprising, for instance, a box-shaped vessel main body comprising anopening in the upper direction and a lid body, and having a constitutionallowing for stacking from top to bottom.

A respective compartment-walled vessel 6 is housed inside each supportvessel 4, a respective CaF₂ crystal 50 is housed inside the eachcompartment-walled vessel 6, a fluoride gas trap layer 5 is formedbetween the each compartment-walled vessel 6 and the each support vessel4.

The support vessel 4 can be formed from general carbon materials suchas, for instance, extrusion-molded articles or CIP-molded articles ofcarbon.

The compartment-walled vessel 6 is a vessel that fills a role for thepurpose of separation so that the CaF₂ crystal 50 does not come directlyinto contact with the fluoride gas trap layer 5, comprises, forinstance, a box-shaped vessel main body comprising an opening in theupper direction and a lid body, and can be formed from general carbonmaterials such as, for instance, extrusion-molded articles or CIP-moldedarticles of carbon.

If the CaF₂ crystal 50 and the fluoride gas trap layer 5 enter intocontact, not only the fluoride gas-adsorbing material adheres onto thesurface of the CaF₂ crystal 50 during the heat-treatment, compromisingthe optical properties of the CaF₂ crystal 50, but a grain boundarystructure of the crystal surface in contact also develops, compromisingthe optical properties; therefore, separating the two parties isimportant.

The fluoride gas trap layer 5 can be formed by filling the fluoridegas-adsorbing material between the compartment-walled vessel 6 and thesupport vessel 4, so as to surround the entire periphery of thecompartment-walled vessel 6, as shown in FIG. 2.

From the point of view of chemical reactions, it is desirable that thefluoride gas-adsorbing material is a material capable of adsorbingeffectively a fluoride gas of a transition metal such as Cr, Fe, Ni andMn, and is one having a vapor pressure that is equal to or lower thanthat of the scavenger PbF₂. Among these, those whereof the vaporpressure is equal to or lower than that of CaF₂ are desirable.Concretely, to be powders, debris and grounds of a fluoride, forinstance, powders, debris or grounds of a fluoride of Ca, or, powders,debris or ground powders of a fluoride of an element of the same familyas Ca, for instance, an alkaline-earth element such as Mg, Sr or Ba, or,a mixture of two or more species thereof, is desirable. Among these, tobe either of powders, debris and grounds of a fluoride of Ca, or amixture of two or more species thereof, is more desirable.

From the point of view of gas trapping ability, the fluoridegas-adsorbing material is preferably a mixture with dispersed sizes soas to allow for fine filling.

From the point of view that a fluoride gas can be adsorbed effectively,the thickness of the fluoride gas trap layer 5 is preferably 5 mm to 200mm, in particular 10 mm to 100 mm, of which in particular 20 mm to 50 mmis more desirable.

While it is desirable for the fluoride gas trap layer 5 to be formed soas to surround the entirety of the periphery of the compartment-walledvessel 6, it may be formed so as to surround a portion thereof.

In addition, the fluoride gas trap layer 5 may be formed into aplurality of layers.

It is adequate that the atmosphere in the heat-treatment, that is tosay, the atmosphere inside the annealing case 3, is a vacuum atmosphereor an inert gas atmosphere such as of argon (Ar). Among them, inert gasatmospheres such as of argon, and among these, atmospheres comprising afluorine gas mixed with/injected into argon gas, are desirable.

In addition, as shown in FIG. 1, housing a scavenger or a fluorinatedagent inside the annealing case 3 is desirable.

It is possible to use as fluorinated agent, for instance, Teflon(registered trademark), acidic ammonium fluoride (NH₄F.HF) or the like,or, lead fluoride, zinc fluoride or the like, or, a substance whereofthe fluorine constituent can be gasified by raising the temperature.

While this fluorinated agent is one that is used in order to preventoxygen and moisture remaining on the surface of the CaF₂ crystal 50 orinside the compartment-walled vessel 6 from reacting with the CaF₂crystal 50, the use is not absolutely needed.

The temperature profile in the heat-treatment step is not limited inparticular. Since the melting point of calcium fluoride is on the orderof 1,370° C. to 1,410° C., heating to a temperature where the CaF₂crystal 50 does not dissolve and maintains the state of a solid whileeach atom constituting the CaF₂ crystal 50 is given sufficient energy tobe moved to a suitable position respectively to cancel an anisotropy dueto a disturbance of the crystal structure is adequate, and thistemperature region is not limited in particular. As a guide, in order tocancel more effectively an anisotropy due to a disturbance of thecrystal structure, raising the temperature to 1,000 to 1,350° C. isdesirable.

While the rate of rise in temperature is not limited in particular, asthere is the necessity of raising the temperature inside the oven sothat the CaF₂ crystal 50 housed inside the compartment-walled vessel 6does not generate damages such as a crack due to a thermal shock,raising the temperature for instance at 10° C./h to 200° C./h isdesirable.

In so doing, first, the temperature inside the oven may be raised to apredetermined target temperature of temperature rise (temperature risestep), then performing at least twice a rise-drop cycle in whichtemperature dropping and temperature rising are carried out alternatelyin the predetermined heat-treatment temperature region (rise-drop cyclestep) and thereafter transitioning to a cooling step.

In the cooling step after the heat-treatment, taking time and coolingslowly is desirable, since distortions are likely to remain inside thecrystal, and in addition, sliding defects are introduced, increasingdislocation or the like, if cooling rapidly. On the other hand,productivity is noticeably lost if too much time is spent. From suchpoints of view, in the cooling step after the heat-treatment, cooling tonear room temperature at a cooling rate of, for instance, 0.1 to 5°C./h, and in particular 0.5 to 1.5° C./h, is desirable.

Then, finally, it suffices that the CaF₂ crystal 50 after heat-treatmentis cut and processed as necessary into a suitable shape. For instance,processing into a shape having as a surface a plane that is parallel tothe (111) plane is sufficient. As a more concrete example, the method ofcutting a CaF₂ crystal 50 presenting a disk shape into a shape having asurface that is parallel to the (111) plane, further surface-grindingthe surface for the purpose of smoothing the surface can be cited.

<Applications>

Since the present fluorite is a homogeneous fluorite, in which there arelittle dislocations and sub-boundary structures, and in particularhaving excellent laser durability, it can be used as, for instance, anapochromatic lens (apochromat), a TV camera lens, a microscope lens, awindow material for infrared analysis, a lens used in semiconductorlithography (stepper and scanner) devices, or other optical lens. Inparticular, since a fluorite in which the homogeneity of the crystal ismacroscopically high, and having excellent laser durability can beobtained, it can be used suitably as a lens material for a highprecision stepper, that is to say, a stepper of an exposure apparatus,or the like, using laser as a light source in the ultraviolet or thevacuum ultraviolet wavelength region such as an ArF (argon fluoride)excimer laser. In addition, having excellent laser durability, thepresent fluorite can be used suitably as a window material of a laserbeam source of ultraviolet or vacuum ultraviolet wavelength region suchas of an ArF excimer laser, or an optical element such as of a resonatormirror.

<Explanation of the Terms>

In the present invention, when “X to Y” (X and Y are any numbers) isstated, unless specified otherwise, it is to include the meaning of “Xor greater but Y or less” along with the meaning of “preferably largerthan X” or “preferably smaller than Y”.

In addition, when “X or greater” (X is any number) is stated, unlessspecified otherwise, it is to include the meaning of “preferably largerthan X”, and when “Y or less” (Y is any number) is stated, unlessspecified otherwise, it is to include the meaning of “preferably smallerthan Y”.

EXAMPLES

Hereinafter, examples and comparative examples according to the presentinvention will be described. However, the present invention is notlimited to the contents described below.

First, evaluation methods for the obtained fluorite will be described.

<Evaluation Method for Laser Durability>

Regarding the laser durability measured as a decrease in transmittancewhen irradiating ArF excimer laser onto a fluorite, in the presentinvention, it was decided to evaluate the laser durability by observingthe decrease in transmittance induced when irradiating a radiation froma higher energy radiation source, that is to say, the absorption of theinduced color center.

Consequently, in the present invention, γ-rays (1.17 MeV and 1.33 MeV)emitted from the radioisotope ⁶⁰Co was irradiated in predetermined dosesonto a radiation source, the color center induced at that moment insidethe crystal was measured with a spectrophotometer to obtain an inducedcolor center absorption spectrum. A negative correlation is known toexist for the relationship between laser durability and γ-ray-inducedcolor center absorption intensity. That is to say, in a crystal withhigh laser durability, the γ-ray-induced color center absorptionintensity is small. The laser durability of the present fluorite can beevaluated from this correlation relationship.

Concretely, both end faces of a fluorite sample were optical polished sothat the planes became parallel, and the optical length (samplethickness) was set to be 30 mm. Such a fluorite sample was retainedinside a dark box, and a dose of 5.4 kGy γ-ray (1.33 MeV) from 60Co wasirradiated in air to induce a color center in the sample. Next, afterirradiation, a recording spectrophotometer (U-4100, Hitachi HighTechnologies) was used rapidly to measure the absorption spectrum ofthis fluorite sample in the UV-visible wavelength region (200 nm to 800nm).

By “absorption” here, the so-called absorption coefficient (valueobtained by taking the natural logarithm of transmittance corrected forthe reflections of the end faces and normalizing with the lengthaccording to the Lambert-Beer's Law; the unit is cm⁻¹) was adopted.

In addition, in order to quantify the induced color center absorptionintensity, the value obtained by integrating the obtained absorptionspectrum in an interval from 200 nm to 800 nm wavelength was used. Thisintegration value is defined as the γ-ray-induced color centerintensity. That is to say, if the laser durability is low (high), theinduced absorption spectrum integration value becomes large (small).

<Evaluation Method for Etch-Pits>

In the present example, with respect to the etch-pit distribution in theCaF₂ crystal, the Voronoi surface areas and the Delaunay distancesdescribed below were defined, the variations (standard deviations)thereof were calculated, whereby the dislocation distribution (etch-pitdistribution) in the CaF₂ crystal was quantitatively evaluated.

(Calculation Methods for the Standard Deviations of Voronoi SurfaceAreas and Delaunay Distances)

1) In order to obtain a clean surface of CaF₂ crystal, it was cleaved orprecision polished in the (111) plane.

Here, the reason for having the CaF₂ (111) plane as the etching surfaceis that a flat surface (that is to say a surface of cleavage) can beobtained easily. In addition, the obtained etch-pit is characterized bythe fact that a pit with a trigonal pyramidal shape constituted by other(111) planes is obtained.

2) Etching of 25° C.×1 hour was performed by immersion in an etchant (7%HCl solution).

3) The etched surface (4 mm in four directions) was photographed with alight microscope, and the image was digitized.

4) Based on the etch-pit image digitized in this way, the etch-pits andportions other than these were processed by binarization (Backgroundremoval, threshold value setting). In addition, dust and scratches otherthan the etch-pits were eliminated. In addition, adjacent, overlappingetch-pits were separated manually and by watershed segmentation.

5) If etch-pits were regularly arranged and adjacent in sub-boundariesor grain boundaries and there were overlaps, each etch-pit was separatedbased on an average neighboring distance. Concretely, the separation wasdone by generating a mesh (Grid) against an etch-pit group that had beenbinarized and recognized as a line.

6) For the etch-pit images adjusted in this way (refer to FIG. 7, FIG.10, FIG. 13 and FIG. 16), Delaunay segmentation and Voronoi segmentationwere performed using an image processing soft (freeware: ImageJ). Thatis to say, for the entirety of the etch-pits within the effective fieldof view, the center-of-gravity point thereof was determined and servedas a generatrix, and with respect to the entirety of the generatrixes inthe image, domain-separation (via Voronoi segmentation) was carried outdepending on which generatrixes the other generatrixes were close to andit is served as a Voronoi diagram (refer to FIG. 9, FIG. 12, FIG. 15 andFIG. 18), each polygonal-shape region that constitutes the Voronoidiagram is served as a Voronoi region and the border line separatingeach region is served as a Voronoi boundary. In addition, a diagramdecomposed newly by linking together the entirety of the generatrixes(these are referred to as a “Delaunay point”) contained in two Voronoiregions that are adjacent through the Voronoi boundary, served as aDelaunay diagram (or, Delaunay diagram; refer to FIG. 8, FIG. 11, FIG.14 and FIG. 17).

Next, the surface area of a Voronoi region (Voronoi figure) in theVoronoi diagram within the effective field of view was defined andcalculated as “Voronoi surface area”, and the standard deviation wascalculated by carrying out statistical processing to evaluate thevariation in the Voronoi surface area.

In addition, the lengths of the edges of each figure (defined as the“Dealunay distances”) in the Delaunay diagram (Dealunay decomposition)were calculated and the standard deviation of the Delaunay distances wasdetermined by carrying out statistical processing to evaluate thevariation.

For the samples of examples and comparative examples, the relationshipbetween the γ-ray-induced color center absorption intensity in thefluorite crystal (alternative evaluation parameter to laser durability)and the standard deviation of the Voronoi surface area or the standarddeviation of the Delaunay distance, are shown in FIG. 3 as well as FIG.4, respectively.

Example 1

A CaF₂ crystal ingot grown by the Bridgman-Stockbarger method (BSmethod) was cut out in the <111> direction, processed into a disk shapewith a size of approximately 80 mm diameter and approximately 30 mmthickness to obtain an as-grown crystal substrate.

In the following examples and comparative examples, the respectivecrystal substrates were collected from equivalent sites of an identicalcrystal ingot.

The crystal substrate obtained in this way was heat-treated using aheat-treatment oven with a constitution shown in FIG. 1 and subsequentlycooled. Likewise for the following comparative examples.

In so doing, a ground powder of CaF₂ crystal (particle sizedistribution: 10 μm to 10 mm) serving as a fluoride gas-adsorbingmaterial was filled between the compartment-walled vessel 6 and thesupport vessel 4 so as to surround the entire periphery of thecompartment-walled vessel 6 to form a 20 mm-thick fluoride gas traplayer 5.

In addition, as shown in FIG. 1, a PbF₂ powder was placed inside theannealing case 3 as a scavenger or a fluorinated agent.

The profile of the heat-treatment step was as follows.

First, at room temperature, the interior of the heat-treatment oven wasreduced in pressure to have a vacuum atmosphere, then, the atmosphereinside the oven was rapidly substituted to Ar gas atmosphere, and thepressure inside the oven was set to 1 atmosphere.

Thereafter, the temperature was raised up to the highest temperature of1,100° C. with a temperature rise time of 36 hours with a heater, andthen the temperature was maintained for 24 hours. Thereafter, cooling toroom temperature took 10 days.

From the crystal substrate obtained by heat-treating in this way, aspecimen (sample) for etch-pit observation was cut-out and cleaved inthe (111) plane in order to obtain a clean surface for observationsurface use.

In addition, a sample for the purpose of evaluating laser durability wascut-out and optical polishing was performed on both (111) plane endsurfaces.

Example 2

A specimen (sample) was obtained in a similar manner to Example 1,except that, in the profile of the heat-treatment step of Example 1, thetemperature was raised up to the highest temperature of 1,000° C. with atemperature rise time of 36 hours, and then the temperature wasmaintained for 24 hours.

Comparative Example 1

A specimen (sample) was obtained in a similar manner to Example 1,except that no ground powder of CaF₂ crystal was filled in Example 1.

Comparative Example 2

A specimen (sample) was obtained in a similar manner to Example 2,except that no ground powder of CaF₂ crystal was filled in Example 2.

TABLE 1 ⁶⁰Co-γ-ray-induced Voronoi surface Delaunay distance colorcenter area standard standard deviation absorption intensity Specimendeviation (μm²) (μm) (×10⁻⁷) Example 1 4312 66.7 2.45 Example 2 185339.9 1.59 Comparative 6205 82.0 5.59 Example 1 Comparative 8003 92.76.13 Example 2

Example 3

A specimen (sample) was obtained in a similar manner to Example 1,except that a mixture from a ground powder of CaF₂ crystal (particlesize distribution: 10 μm to 10 mm) and a powder of lead fluoride(particle size: approximately 50 μm) mixed at a mass proportion of 99:1was filled between the compartment-walled vessel 6 and the supportvessel 4 so as to surround the entire periphery of thecompartment-walled vessel 6 to form a 20 mm-thick fluoride gas traplayer 5

Examples 4 to 8

Specimens (samples) were obtained in a similar manner to Example 1,except that, in the profile of the heat-treatment step of Example 1, thetemperature was raised up to the highest temperatures of 950 to 1,200°C. with temperature rise times of 30 to 36 hours, and then thetemperature was maintained for 24 hours.

Comparative Examples 3 to 5

Specimens (samples) were obtained in a similar manner to ComparativeExample 1, except that, in the profile of the heat-treatment step ofComparative Example 1, the temperature was raised up to the highesttemperatures of 950 to 1,200° C. with temperature rise times of 30 to 36hours, and then the temperature was maintained for 24 hours.

(Discussion)

When heat-treating a CaF₂ crystal grown similarly to prior art, if thecases where heat-treatment was conducted by providing a fluoride gastrap layer through compartment walls in the periphery of the fluoritecrystal (Examples) are compared to the cases where heat-treatment wasconducted without providing a fluoride gas trap layer (ComparativeExamples), they differ greatly regarding both the standard deviation ofthe Voronoi surface areas and the standard deviation of the Delaunaydistances as shown in FIG. 3 and FIG. 4, and fluorite crystals havingexcellent laser durability were found to be obtained in the cases whereheat-treatment was conducted by providing a fluoride gas trap layer.

In addition, focusing on the standard deviation of the Voronoi surfaceareas (FIG. 3), in the cases where heat-treatment was conducted byproviding a fluoride gas trap layer through compartment walls in theperiphery of the fluorite crystal (Examples), it was found that thestandard deviations of the Voronoi surface areas were 6,000 μm² or lessand the color center absorption intensity (alternative evaluationparameter to laser durability) increased noticeably. From such points ofview, it can be considered that for the standard deviation of theVoronoi surface areas of the present fluorite to be 6,000 μm² or less isdesirable, and in particular to be 4,000 μm² or less, of which inparticular 3,000 μm² or less is all the more desirable.

Meanwhile, focusing on the standard deviation of the Delaunay distance(FIG. 4), in the cases where heat-treatment was conducted by providing afluoride gas trap layer through compartment walls in the periphery ofthe fluorite crystal (Examples), it was found that the standarddeviations of the Delaunay distances were 80 μm or less and the colorcenter absorption intensity (alternative evaluation parameter to laserdurability) increased noticeably. From such points of view, it can beconsidered that for the standard deviation of the Voronoi surface areasof the present fluorite to be 80 μm or less is desirable, and inparticular to be 60 μm or less, of which in particular 50 μm or less isall the more desirable.

The invention claimed is:
 1. A fluorite, wherein the standard deviationof the surface areas of the Voronoi regions in a diagram from theVoronoi segmentation of the distribution of etch-pits in the (111) planeis 6,000 μm² or less, and, the standard deviation of the distances ofthe Delaunay edges in a diagram from the Delaunay segmentation of thedistribution of etch-pits of the (111) plane is 80 μm or less whenetching of 25° C.×1 hour is performed by immersion in 7% HCl solution,the etched surface is photographed with a light microscope, and theimage is digitized, based on the etch-pit image digitized in this way,the etch-pits and portions other than these are processed bybinarization, whereby Delaunay segmentation and Voronoi segmentation areperformed.